The height-to-side ratio of the images in the case of Cinemascope films is generally 1:2.35. That projection format corresponds to the psychological visual perception of a human being better than other narrower formats. Problems are incurred however when recording the originals on the conventional 35 mm-wide film format as the resulting images become too narrow and too small to be able to achieve good results upon projection. For that reason the images are squeezed horizontally by a factor of 2 upon recording. That affords images of normal size on conventional film material, but that squeeze effect has to be reversed upon projection. That is implemented by means of anamorphic objectives. Optical systems of that kind, also referred to as anamorphic lenses, primarily differ from conventional objectives by virtue of the fact that the focal length in a horizontal direction differs markedly from that in the vertical direction. The ratio of those two focal lengths is also referred to as the anamorphic factor. If the horizontal focal length is precisely half that of the vertical focal length, the desired effect of image expansion occurs in the horizontal. If such an anamorphic lens is used to project for example a Cinemascope film, it completely eliminates the squeeze or compression effect introduced upon recording and produces an undistorted image on the screen.
It is known for anamorphic objectives to be constructed in two different ways. Both approaches are based on a corrected main objective which is supplemented by an objective which is entirely made up of unitarily oriented cylindrical lenses.
An anamorphic objective in accordance with the one proposal is described for example in U.S. Pat. No. 2,933,017. Therein, the additional objective which has zero refractive power because of the unitary orientation of the cylindrical lenses in one direction is positioned between the main objective and the film plane. Such an objective is compact and suitable for projection with low aperture values. In the case of high apertures or with large projection angles however that design principle fails, as already explained in U.S. Pat. No. 3,924,933.
The other proposal involves positioning the cylindrical additional objective in front of the main objective. Structures involving that principle are described in U.S. Pat. Nos. 1,962,892 and 2,821,110. Anamorphic objectives of that type are suitable both for wide-angle uses and also for high-aperture uses. Such objectives however are very bulky and heavy. The recommendation therefore for example in U.S. Pat. No. 2,731,883 is to use anamorphic objectives of that kind only in the case of anamorphic factors of less than 1.5 as excessive residual aberration phenomena occur with greater anamorphic factors.
Irrespective of those disadvantages, most projection anamorphic lenses for Cinemascope projection with an anamorphic factor of 2.0 are based on that principle. The excessive residual aberration phenomena are tolerated but they do result in a considerable worsening of image quality.
Spherical attachments for the cylindrical attachments as are described for example in U.S. Pat. No. 2,956,475, German patent specification No 971 992, French No 1 082 780 and U.S. Pat. No. 3,682,533, can make no contribution to improving the imaging power of projection anamorphic lenses. Those spherical attachments permit a focusing action without thereby simultaneously causing a change in the anamorphic factor.
Swiss patent specification No. 350 813, U.S. Pat. No. 3,517,984, German G 84 27 849.8, German patent specification No. 34 36 913 and German patent specification 36 29 438 put the focusability of anamorphic objectives at the center.
Thus however different the two design principles for anamorphic objectives may be, nonetheless they both have similar quality problems in terms of image quality.
Anamorphic systems which are constructed on the basis of corrected main objectives again and again result in anamorphic lenses which at the center of the image have a corrected spherical aberration and no astigmatism, and in the direction in perpendicular relationship to the cylinder axes they are free of coma, image field curvature and astigmatism. However severe curvature of the sagittal image shell always occurs in the direction of the cylinder axes, whereas the meridional is flattened. That astigmatism makes itself disturbingly noticeable upon Cinemascope projection, in a vertical direction. Here the image becomes blurred and fuzzy. The fact that the drop in resolution occurs in particular upon imaging of the radial structures is quite particularly unpleasant as the image is expanded in that direction. Here a particularly good level of imaging quality should actually exist in order to achieve a uniformly good projection result. It can be demonstrated by theory that this image defect which is typical of anamorphic lenses with cylindrical lenses is inevitable if the usual known design principles are applied. The introduction of additional spherical or crossed cylindrical lenses, as described for example in EP 0 388 704 and DE 41 04 684 only immaterially reduce such aberration phenomena. A lasting improvement can be achieved only by a reduction in the anamorphic factor or by a marked increase in structural length. Both are out of the question in terms of Cinemascope projection. On the one hand the anamorphic factor of 2.0 is fixedly preset while on the other hand the increase in structural length at the same time signifies a cut in the projection angle as the objective diameter cannot be increased just as desired.
In the case of anamorphic systems the occurrence of the excessive residual image defects can be avoided only if novel basic principles for design are used.
Such an attempt was undertaken in accordance with U.S. Pat. No. 4,362,366. Here, the two design principles described in the opening part of this specification were used at the same time in one objective. That made it possible to reduce the anamorphic factor for each of the two cylindrical sub-systems. The anamorphic factor of the overall system is here the product of the sub-systems. Astigmatism in a vertical direction could be controlled by virtue of the crossed installation of the cylindrical systems. At the same time however the arrangement involved higher-order aberration phenomena, in particular clover leaf defects, with the result that that approach cannot be used for aperture ratios of over 1:5.6.